[PATCH v3 2/2] kho: history: track kexec boot counter

Breno Leitao
2026-01-08T16:41:28Z

Track and display the number of kexec boots since the last cold reboot.

This extends the previous kernel release tracking by adding a counter
that increments with each kexec boot. The counter provides visibility
into the kexec chain depth, which is useful for understanding boot
history in production environments.

Add a new property "kexec-count" in KHO FDT alongside the existing
"previous-release" property. The counter is:

- Initialized to 0 on cold boot (when kho_in is first instantiated)
- Incremented by 1 on each subsequent kexec
- Printed alongside the previous kernel release version

The counter is stored as a 32-bit unsigned integer in FDT format.

This is different than counter for LUO, given KHO can be used
independently from LUO.

Also WARN() if KHO_PROP_PREVIOUS_RELEASE doesn't exist, because it
must exist if KHO_PROP_PREVIOUS_RELEASE exists.

Signed-off-by: Breno Leitao <leitao@debian.org>
Suggested-by: Pasha Tatashin <pasha.tatashin@soleen.com>
---
include/linux/kho/abi/kexec_handover.h | 3 +++
kernel/liveupdate/kexec_handover.c | 16 +++++++++++++++-
2 files changed, 18 insertions(+), 1 deletion(-)

diff --git a/include/linux/kho/abi/kexec_handover.h b/include/linux/kho/abi/kexec_handover.h
index f4f31e8f575b..9c4fc4c6a212 100644
--- a/include/linux/kho/abi/kexec_handover.h
+++ b/include/linux/kho/abi/kexec_handover.h
@@ -87,6 +87,9 @@
/* The FDT property to track previous kernel (kexec caller) */
#define KHO_PROP_PREVIOUS_RELEASE "previous-release"

+/* The FDT property to track number of kexec counts so far */
+#define KHO_PROP_KEXEC_COUNT "kexec-count"
+
/**
* DOC: Kexec Handover ABI for vmalloc Preservation
*
diff --git a/kernel/liveupdate/kexec_handover.c b/kernel/liveupdate/kexec_handover.c
index b2d57868d22f..34cfc98076b8 100644
--- a/kernel/liveupdate/kexec_handover.c
+++ b/kernel/liveupdate/kexec_handover.c
@@ -1248,6 +1248,7 @@ struct kho_in {
phys_addr_t scratch_phys;
phys_addr_t mem_map_phys;
char previous_release[__NEW_UTS_LEN + 1];
+ u32 kexec_count;
struct kho_debugfs dbg;
};

@@ -1319,6 +1320,7 @@ static __init int kho_out_fdt_setup(void)
{
void *root = kho_out.fdt;
u64 empty_mem_map = 0;
+ u32 kexec_count;
int err;

err = fdt_create(root, PAGE_SIZE);
@@ -1329,6 +1331,10 @@ static __init int kho_out_fdt_setup(void)
sizeof(empty_mem_map));
err |= fdt_property_string(root, KHO_PROP_PREVIOUS_RELEASE,
init_uts_ns.name.release);
+ /* kho_in.kexec_count is set to 0 on cold boot */
+ kexec_count = kho_in.kexec_count + 1;
+ err |= fdt_property(root, KHO_PROP_KEXEC_COUNT, &kexec_count,
+ sizeof(kexec_count));
err |= fdt_end_node(root);
err |= fdt_finish(root);

@@ -1443,15 +1449,23 @@ void __init kho_memory_init(void)
static int __init kho_print_previous_kernel(const void *fdt)
{
const char *prev_release;
+ const u32 *count_ptr;
int len;

prev_release = fdt_getprop(fdt, 0, KHO_PROP_PREVIOUS_RELEASE, &len);
if (!prev_release || len <= 0)
return -ENOENT;

+ /* Read the kexec count from the previous kernel */
+ count_ptr = fdt_getprop(fdt, 0, KHO_PROP_KEXEC_COUNT, &len);
+ if (WARN_ON(!count_ptr || len != sizeof(u32)))
+ return -ENOENT;
+ kho_in.kexec_count = *count_ptr;
+
strscpy(kho_in.previous_release, prev_release,
sizeof(kho_in.previous_release));
- pr_info("exec from: %s\n", kho_in.previous_release);
+ pr_info("exec from: %s (count %u)\n", kho_in.previous_release,
+ kho_in.kexec_count);

return 0;
}

--
2.47.3